膿包行了 Do Not Cry Master
行者道：「莫哭，莫哭，一哭便膿包行了。他這報信，必有幾分虛話，只是要我們著意留心，誠所謂：『以告者，過也。』你且下馬來坐著。」行者道：「莫哭，莫哭，一哭便脓包行了。他这报信，必有几分虚话，只是要我们着意留心，诚所谓：『以告者，过也。』你且下马来坐着。」
逐句解 Line by line
Pilgrim said, "Do not cry, do not cry. One cry and you are a wash-out.
膿包 is literally a boil full of pus, and as an insult it means a weakling, a wash-out, somebody who bursts at a touch. It is still current and still rude. Pilgrim's bedside manner has not improved since he picked his master out of the grass.
There is bound to be some falsehood in this report of his — it is only meant to make us fix our minds and take care, as the saying has it: the one who tells you is the one at fault. Get down off the horse and sit a while."
著意留心 has 著 as zhuó — to fasten the attention, the same phrase the chapter's opening poem used. 坐著 four characters later has the same character as zhe, the durative. And 以告者，過也 is a classical tag set off in its own quotation marks: the fault lies with the one who reports it. Pilgrim is quoting a book at a man who is crying.
Pilgrim's comfort consists of calling his master a burst boil, telling him the warning was probably exaggerated, and quoting a classical tag. It is not what anybody would want and it is entirely sincere.
The quotation is the strange part. Having just been told by Venus in person that the danger is real, he produces a saying about informants being at fault — which is not an argument so much as a way of changing the subject.
And then, having dismissed the whole thing, he tells everybody to get off the horse and sit down. The instruction gives the game away: he is about to do something, and he does not want his master on a horse when he does it.